A pulmonary embolism occurs when an embolus become lodged in lung arteries, thus blocking blood flow to lung tissue. An embolus is usually a blood clot, known as a thrombus, but may also comprise fat, amniotic fluid, bone marrow, tumor fragments, or even air bubbles that block a blood vessel. Unless treated promptly, a pulmonary embolism maybe fatal.
A pulmonary embolism may be difficult to detect because signs and symptoms may vary depending on the severity of the occurrence. For instance, a pulmonary embolism may be confused with a heart attack, pneumonia, hyperventilation, congestive heart failure or a panic attack. In other cases, there may be no symptoms at all.
A physician will sometimes first eliminate the occurrence of other lung diseases before determining that the symptoms, if any, are caused by a pulmonary embolism. Traditional diagnostic methods of testing involve blood tests, chest X-rays, and electrocardiograms. These methods may typically be more effective in ruling out other possible problems than for actually diagnosing a pulmonary embolism. For example, a chest x-ray may reveal subtle changes in the blood vessel patterns after an embolism and signs of pulmonary infarction. However, chest x-rays may show normal lungs even when an embolism is present. Similarly, an electrocardiogram may show abnormalities that are mainly useful in establishing the possibility of a pulmonary embolism.
As a pulmonary embolism alters the ability of the lungs to oxygenate the blood and to remove carbon dioxide from the blood, one method of diagnosing the condition involves taking a specimen of arterial blood and measuring the partial pressure of oxygen and carbon dioxide in the arterial blood (i.e., an arterial blood gas analysis). Although a pulmonary embolism often causes abnormalities in these measurements, an individual finding or combination of findings from the arterial blood gas analysis does not necessarily provide a reliable way to exclude or a specific way of diagnosing a pulmonary embolism. For instance, some patients with a documented pulmonary embolism have normal oxygen and carbon dioxide contents of the arterial blood. Accordingly, the arterial blood analysis may not reliably include or exclude the diagnosis of a pulmonary embolism.
The blood D-dimer assay is another diagnostic method that has become available for commercial use. A D-dimer protein fragment is typically formed when fibrin is cleaved by plasmin and therefore produced naturally whenever clots form in the body. However, many studies have shown a D-dimer assay may produce a high degree of false positives.
In an attempt to increase the accuracy of diagnostic procedures for pulmonary embolisms, physicians have recently turned to methods that can produce an image of a potentially afflicted lung. One such method is a nuclear perfusion study that involves the injection of a small amount of radioactive particles into a vein. The radioactive particles then travel to the lungs where they highlight the perfusion of blood in the lung based upon whether they can penetrate a given area of the lung. However, one possible drawback with this method is that an abnormal scan does not necessarily mean that a pulmonary embolism is present.
Pulmonary angiograms are another means of diagnosing a pulmonary embolism. During a pulmonary angiogram, a catheter is threaded into the pulmonary artery so that iodine dye can be injected into the bloodstream. The dye flows into the regions of the lung and is imaged using x-ray technology, which may indicate a pulmonary embolism as a blockage of flow in an artery. Pulmonary angiograms may be useful in diagnosing pulmonary embolisms but often presents health risks and can be expensive. Spiral volumetric computed tomography is another diagnostic tool that has recently been proposed as a possibly less invasive test for detecting a pulmonary embolism. This procedure's reported sensitivity has varied widely, however, it may only be useful for diagnosing an embolism in the central pulmonary arteries, as it may be relatively insensitive to clots in more remote regions of the lungs.
The above-discussed pulmonary vascular imaging tests have several disadvantages in common. Many of the tests require ionizing radiation and invasiveness of, at a minimum, an intravenous catheter. Some tests also typically involve costs of more than $1,000 for the patient, take more than two hours to perform, and require special expertise such as a trained technician to perform the tests and acquire the images and a board-certified radiologist to interpret the images. Notably, many of the tests may not be completely safe for patients who are pregnant. As a result of these shortcomings, many of the imaging procedures currently used are not available in many outpatient clinic settings.